Method and apparatus for distributing satellite tracking information

ABSTRACT

A method and apparatus for distributing satellite tracking data to a remote receiver. At least a portion of the satellite tracking data is extracted from memory and is formatted into a format prescribed by a remote receiver. The formatted data is transmitted to the remote receiver via a distribution network.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of co-pending U.S.patent application Ser. No. 09/875,809, filed Jun. 6, 2001 (AttorneyDocket GLBL/016), which is herein incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention generally relates to generating satellitetracking information for earth orbiting satellites. More specifically,the invention relates to a method and apparatus for generating anddistributing satellite tracking information through a network orcommunications link.

[0004] 2. Description of the Related Art

[0005] A positioning receiver for the Global Positioning System (GPS)uses measurements from several satellites to compute a position. Theprocess of acquiring the GPS radio signal is enhanced in speed andsensitivity if the GPS receiver has prior access to a model of thesatellite orbit and clock. This model is broadcast by the GPS satellitesand is known as an ephemeris or ephemeris information. Each satellitebroadcasts its own ephemeris once every 30 seconds. Once the GPS radiosignal has been acquired, the process of computing position requires theuse of the ephemeris information.

[0006] The broadcast ephemeris information is encoded in a 900 bitmessage within the GPS satellite signal. It is transmitted at a rate of50 bits per second, taking 18 seconds in all for a complete ephemeristransmission. The broadcast ephemeris information is typically valid for2 to 4 hours into the future (from the time of broadcast). Before theend of the period of validity the GPS receiver must obtain a freshbroadcast ephemeris to continue operating correctly and produce anaccurate position. It is always slow (no faster than 18 seconds),frequently difficult, and sometimes impossible (in environments withvery low signal strengths), for a GPS receiver to download an ephemerisfrom a satellite. For these reasons it has long been known that it isadvantageous to send the ephemeris to a GPS receiver by some other meansin lieu of awaiting the transmission from the satellite. U.S. Pat. No.4,445,118, issued Apr. 24, 1984, describes a technique that collectsephemeris information at a GPS reference station, and transmits theephemeris to the remote GPS receiver via a wireless transmission. Thistechnique of providing the ephemeris, or equivalent data, to a GPSreceiver has become known as “Assisted-GPS”. Since the source ofephemeris in Assisted-GPS is the satellite signal, the ephemerisinformation remains valid for only a few hours. As such, the remote GPSreceiver must periodically connect to a source of ephemeris informationwhether that information is received directly from the satellite or froma wireless transmission. Without such a periodic update, the remote GPSreceiver will not accurately determine position.

[0007] The deficiency of the current art is that there is no source ofsatellite trajectory and clock information that is valid for longer thana few hours into the future, and it can be expensive to send theephemeris information repeatedly to the many remote devices that mayneed it. Moreover, mobile devices may be out of contact from the sourceof the Assisted-GPS information when their current ephemeris becomesinvalid.

[0008] Therefore, there is a need in the art for a method and apparatusfor providing satellite trajectory and clock information that is validfor an extended period into the future, e.g., many days into the future.

SUMMARY OF THE INVENTION

[0009] The present invention is a method and apparatus for generatingsatellite tracking data (STD) that is valid for extend periods of timeinto the future, i.e., long term STD or LT-STD. The STD may containfuture satellite trajectory information and/or satellite clockinformation. The STD is derived by receiving at one or more satellitetracking stations the signals from at least one satellite anddetermining satellite tracking information (STI) from the receivedsignals. STI contains present satellite orbit trajectory data andsatellite clock information.

[0010] The STD may be provided to a remote satellite signal receiver viaa network or communications system. The satellite system may include theglobal positioning system (GPS), GLONASS, GALILEO, or other satellitesystems that may use STD to enhance the performance of the receiver. Byusing the LT-STD, a remote receiver may accurately operate for dayswithout receiving an update of the broadcast ephemeris information asnormally provided from the satellites.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] So that the manner in which the above recited features of thepresent invention are attained and can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe had by reference to the embodiments thereof which are illustrated inthe appended drawings.

[0012] It is to be noted, however, that the appended drawings illustrateonly typical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

[0013]FIG. 1 depicts a system for creating and distributing satellitetracking data (STD) to remote GPS receivers;

[0014]FIG. 2 depicts a method for forming the STD from the satellitemeasurements made at satellite tracking stations;

[0015]FIG. 3 depicts a timeline of STD data that conforms to thebroadcast ephemeris format models as described in ICD-GPS-200C yet spansmany hours;

[0016]FIG. 4 depicts a flow diagram of a method that uses a leastsquares estimation technique to update parameters in an orbit trajectorymodel;

[0017]FIG. 5 depicts the error in the orbit model derived from the STD,and compares the error to the error in the broadcast ephemeris;

[0018]FIG. 6 depicts an example of a data table that could be used in anSTD database.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019]FIG. 1 depicts a block diagram of a system 100 for creating anddistributing satellite tracking data (STD). The satellite system mayinclude the global positioning system (GPS), GLONASS, GALILEO, or othersatellite systems that may use STD to enhance the performance of thereceiver. The following disclosure uses GPS as an illustrative systemwithin which the invention operates. From the following disclosure,those skilled in the art will be able to practice the invention inconjunction with other satellite systems.

[0020] A network of GPS tracking stations 102 is used to collectmeasurement data from the GPS satellites 104. Such a network isdescribed in detail in U.S. patent application Ser. No. 09/615,105,filed Jul. 13, 2000. The network could comprise several trackingstations that collect satellite tracking information (STI) from all thesatellites in the constellation, or a few tracking stations, or a singletracking station that only collects STI for a particular region of theworld. An STD collection and computation server 106 collects andprocesses the measurement data (this measurement data is referred toherein as satellite tracking information (STI) that includes at leastone of: code phase measurements, carrier phase measurements, Dopplermeasurements, or ephemeris data). In the preferred embodiment,measurement data is obtained from both the L1 and L2 frequencies onwhich the GPS satellites transmit. Alternative embodiments may use onlyone of these frequencies, and/or other frequencies used by othersatellite systems or by future versions of the GPS system. The server106 produces: 1) accurate satellite tracking data (STD) (e.g., atrajectory of each satellite and/or a clock offset measurement) duringthe data collection period, 2) a prediction of the future STD of eachsatellite, and 3) models that match the future STD of each satellite.The server 106 comprises a central processing unit (CPU) 118, supportcircuits 122, and memory 120. The CPU 118 may be any one of the manyCPUs available on the market to perform general computing.Alternatively, the CPU may be a specific purpose processor such as anapplication specific integrated circuit (ASIC) that is designed toprocess satellite tracking information. The support circuits 122 arewell known circuits such as clock circuits, cache, power supplies andthe like. The memory 120 may be read only memory, random access memory,disk drive storage, removable storage or any combination thereof. Thememory 120 stores executable software, e.g., LT-STD software 124, that,when executed by the CPU 118, causes the system 100 to operate inaccordance with the present invention.

[0021] The set of satellite trajectory and clock data produced by theLT-STD software 124 constitutes the STD information, and is stored in anSTD database 108. A distribution server 110 accesses the database 108 togather the most recent set of data, formats the data using thetrajectory conversion software 111 according to the relevant interfacestandard, and distributes the formatted data to GPS devices 112 thatrequire satellite orbit information. The distribution process may be bysome form of wireless communications system 114, or over the Internet116, or a combination of both, or by some other means of communication.Once the GPS devices 112 have received the orbit data, they may operatecontinually for many days without needing to download fresh broadcastephemeris from the satellites or from any other source. The orbit datadistributed to the GPS devices may be in the same format as thebroadcast ephemeris or may be some other model format that is defined bythe GPS device. Herein this orbit data is generally referred to as asatellite tracking model (STM). The loading of the STM into the GPSreceiver can be accomplished in many ways. Using the cradle for apersonal digital assistant (PDA), direct connection to a network, or awireless technology, such as Bluetooth or a cellular network, are a fewexamples of how the ephemeris data can be transferred to the receiver.The transmission is generally accomplished by broadcasting the LT-STD(or a model representing all or a portion of the LT-STD) withoutknowledge of the specific location of the GPS receiver. As such, thedistribution server does not require the GPS receiver to send anyinformation through the network to the distribution server.

[0022] Since GPS is a ranging system in and of itself, the datatransmitted by the GPS satellites can be used to determine the range,range-rate and clock offsets to the GPS satellites from a set oftracking stations. This set of observations generated by the trackingstations 102 is used in the orbit determination process, and in theestimation of the satellite clock characteristics. The set of monitoringstations 102 could be a single station, a public network such as theContinuously Operating Reference System (CORS), or a privately ownedand/or operated network.

[0023]FIG. 2 illustrates the preferred embodiment of a process forcomputing LT-STD. The process begins at step 202 with the collection ofsatellite measurements from the network of tracking stations.Measurements such as code phase, (CP), carrier phase (CPH), and Dopplermay be used for GPS satellite tracking information. At step 204, themeasurements are used to compute the satellite trajectories and clockoffsets over the periods during which the data was collected. This stepis performed using standard GPS processing techniques and softwarepackages well known in the art. Examples of this type of software areGIPSY from the Jet Propulsion Laboratory (JPL), GEODYN from NASA GoddardSpace Flight Center (GSFC), and the commercial product, MicroCosm, fromVan Martin Systems.

[0024] At step 206, the satellite trajectories and clock offsets fromstep 204 are propagated into the future with the same software package,using standard orbit models, such as gravity, drag, solar radiationpressure, tides, third body effects, precession, nutation, and otherconservative and non-conservative forces effecting the satellitetrajectory. These are normally the same force models that are used inthe estimation of the satellite orbits during the data fit interval. Asubset of these models, such as those for drag and solar radiationpressure, are adjusted during the orbit estimation process described instep 204 to best fit the trajectory. This combination of known andestimated force models parameters is used in the propagation 206 toprovide the propagated orbit for time outside the data fit interval. Theclock offsets for GPS satellites are typically very small, and changelinearly over time. These clock offsets are propagated into the futureusing standard models, such as a second order model containing clockoffset, drift, and drift rate.

[0025] At step 208, the propagated satellite trajectories and/or clockoffsets are stored as STD in a database. At step 210, the trajectoryconversion software converts the LT-STD data into a model and formatexpected by the GPS device to which the model is to be provided. At step212, the prescribed model or information is output. For use withexisting GPS receivers, the preferred embodiment of the model is the GPSephemeris model as described in ICD-GPS-200 and an ephemeris model isgenerated from the LT-STD for each 4 hour period as illustrated in thetimeline 300 of FIG. 3, i.e., a different model 301, 302 and so on isgenerated for each six hour period. As such, the plurality of models301, 302 and so on cumulatively span the PATENT length of the availableLT-STD.

[0026] In an alternate embodiment, at step 204, the satellitetrajectories and clock offsets may be estimated using the data broadcastby the satellites and the standard equations given in ICD-GPS-200c.

[0027] The orbit model is a mathematical representation of the satellitetrajectory that describes the trajectory as a function of a small numberof variables and eliminates the need to provide satellite positionvectors explicitly as a table of time vs. satellite positions. Anexample of an ephemeris model is the classic six element Keplerianorbital model. Although this model lacks long term accuracy, it is afunctional ephemeris model for providing satellite trajectoryinformation as a function of a small number of variables. In thepreferred embodiment, the model used to describe the trajectory is GPSstandard ephemeris, specified in ICD-GPS-200c, following the sameconventions and units. This is the preferred method to provide maximumcompatibility with existing GPS receivers. However, other orbit modelscould also be used to represent the satellite trajectory. Orbit modelscan be selected to provide increased accuracy, longer duration fits,more compact representation of the trajectory, or other optimizationsrequired in an application.

[0028] This invention is different from the current art in that theorbit model provided to the GPS device is not the ephemeris databroadcast by the GPS satellites. Current art downloads the ephemerisbroadcast from the GPS satellites and retransmits that data to GPSdevices. In this invention, the broadcast ephemeris data is not requiredat any stage and is not used in the preferred implementation.

[0029] The broadcast ephemeris data provided by the GPS satellites covera specific time period (typically 4 hours) and the end of that time theinformation becomes unusable. For example, if a device receives abroadcast ephemeris that will expire in 5 minutes, the device would needthe new broadcast ephemeris before operating outside that 5 minuteinterval. With this invention, the STD may be formatted for the timeperiod required by the device. This time period may be for the currenttime forward or may be for some time interval in the future. Forexample, a device may request orbit information in the standard GPSephemeris format for the current time. In this case, the ephemerisprovided to the device would be valid for the next 6 hours. The devicecould request orbit information for the next 12 hours in the standardGPS format which could be supplied as two six hour ephemeris orbitmodels. In addition, different orbit models and formats that supportdifferent accuracies and standards can be generated from the LT-STD.

[0030] Fitting the LT-STD to the desired orbit model can be accomplishedin a number of mathematical methods. The preferred embodiment is aleast-squares fit of the orbit model parameters to the trajectory data.Other methods, such as Kalman filters or other estimators can also beused to obtain the orbit model parameters that best fit the trajectorydata. These techniques of fitting data to orbit models are well known topeople skilled in the art of orbit determination and orbit modeling.

[0031] The least squares technique provides an optimal fit of thetrajectory data to the orbit model parameters. FIG. 4 depicts a flowdiagram of a method of generating an orbit model using a least squaresestimation technique. One embodiment of LT-STD is a table representationof time, position, and clock offset for each satellite, as shown in FIG.6. The time, position, and clock offset can be in any time/coordinatesystem. For the purpose of simplicity and illustration, thetime/coordinate system is GPS time and Earth-Centered-Earth-Fixed (ECEF)position in the World Geodetic Survey 1984 (WGS-84) reference frame.

[0032] At step 402, the STD for the desired time interval is extractedfrom the STD database. The orbit model parameters are initialized to theorbit model values obtained by a similar process for the previousinterval. This guarantees that the initial orbit model parameters are agood fit at least for the beginning of the desired time interval. Therest of the process 400 will ensure that the parameters are adjusted sothat they become a good fit for the entire time interval.

[0033] In the preferred embodiment there are 15 orbital parameters to beadjusted:

[0034] Square root of semi-major axis (meters^ ½ )

[0035] Eccentricity (dimensionless)

[0036] Amplitude of sine harmonic correction term to the orbit radius(meters)

[0037] Amplitude of cosine harmonic correction term to the orbit radius(meters)

[0038] Mean motion difference from computed value (radians/sec)

[0039] Mean anomaly at reference time (radians)

[0040] Amplitude of cosine harmonic correction term to the argument oflatitude (radians)

[0041] Amplitude of sine harmonic correction term to the argument oflatitude (radians)

[0042] Amplitude of cosine harmonic correction term to the angle ofinclination (radians)

[0043] Amplitude of sine harmonic correction term to the angle ofinclination (radians)

[0044] Longitude of ascending node of orbit plane at weekly epoch(radians)

[0045] Inclination angle at reference time (radians)

[0046] Rate of inclination angle (radians/sec)

[0047] Argument of perigee (radians)

[0048] Rate of right ascension (radians/sec)

[0049] Although it will be readily apparent that more terms may be used,for better fits, or, fewer terms may be used for a more compact model.

[0050] At step 404, the orbit model is used to predict what thetrajectory would be, the predicted data is denoted the “Model TrajectoryData” (MTD). If the model were perfect, the MTD would coincide exactlywith the STD. At step 406, the MTD and OTD are compared to see howclosely the orbit model fits the orbit data. In the preferredembodiment, the comparison step 406 is performed by summing the squaresof the differences between each trajectory point in the OTD and thecorresponding point in the MTD, and comparing the resulting sum to athreshold. If the fit is “good”, the model parameters are deemed “good”and the process stops at step 410. If the fit is not good then the modelparameters are adjusted at step 408. There are many techniques wellknown in the art for adjusting model parameters to fit data. Forexample, in FIG. 5, the six-hour ephemeris model was adjusted to fit sixhours of OTD using a subspace trust region method based on theinterior-reflective Newton method described in Coleman, T. F., and Y.Li, “On the convergence of reflective Newton methods for large scalenonlinear minimization subject to bounds”, Mathematical Programming,Vol. 67, Number 2, pp. 189-224, 1994, and Coleman, T. F., and Y. Li, “Aninterior, trust region approach for nonlinear minimization subject tobounds”, SIAM Journal on Optimization, Vol. 6, pp. 418-445, 1996. Thereare standard computer packages, e.g., MATLAB Optimization Toolbox, thatmay be used to implement these methods.

[0051] Steps 404, 406 and 408 are repeated until the model parametersare found that fit the OTD well.

[0052] When fitting an orbit model to trajectory data, there are manychoices of which orbit model to choose. The preferred embodiment is touse orbit models with parameters that have been defined in well-knownstandards. In one embodiment, the ephemeris parameters defined in theGPS interface control document, ICD-GPS200c, are used. The ICD-GPS-200cdefinition includes a bit that specifies a 4-hour fit or a 6-hour fit.Typically, the satellite data is broadcast in 4-hour fits and, by thetime this data is obtained by the observer of the satellite, the data isoften near the end of its fit interval. In one embodiment of the currentinvention, sequential 6 hour windows of STD are used to create 6-hourephemeris models, using the technique described in FIG. 4 and theaccompanying text. This produces a set of ephemeris models asillustrated in FIG. 3. Although these particular 6-hour models are notavailable without this invention, the models nonetheless are definedusing standard parameters (i.e. ICD-GPS-200c) and will be understood byany device that was designed to be compatible with said standard.

[0053] Alternatively, the transmission time for the model may bedynamically determined in response to various transmission networkcharacteristics, e.g., cellular telephone rate structures, datatransmission bandwidths, low network utilization periods, low networkcongestion periods and the like. Thus, the invention determines presentvalue of the specific characteristics and compares the present value toa threshold. In response to the comparison, the invention will transmitor not transmit the model. For example, the invention may monitor thenetwork traffic and determine the least congested time to transmit themodel. Many wireless networks have time varying rates. For example,cellular telephone use is often less expensive on weekends compared tomid-week rates. A useful embodiment of the current invention is tocreate a satellite tracking model that is valid for the period betweeninexpensive rates (example: valid from one Saturday to the next), andtransmit the model during the time that the rate is inexpensive. Assuch, the model is transmitted for less cost than if the models weretransmitted during a peak rate period. Also, or as an alternative, onemay define and send the model to coincide with periods of low data useon the network—whether the network is wireless or not (e.g. theinternet). Those skilled in the art will realize that many othertransmission time optimization characteristics can be used to determinewhen it is best to transmit the model to the receiver(s).

[0054]FIG. 5 shows an example of Satellite Tracking Data (STD) that wasgenerated for a time interval of greater than six hours. Then, using thetechnique described by FIG. 4 and accompanying text, parameters of anICD-GPS-200c ephemeris model were adjusted to give a best fit to 6 hoursof the STD. The orbit modeled by this 6-hour ephemeris was then comparedto the true trajectory and, for comparison, the true trajectory was alsocompared to the orbit modeled by the broadcast ephemeris. The resultsare shown in FIG. 5, illustrating how the broadcast ephemeris losesvalidity while the ephemeris created by this invention maintains itsvalidity with approximately one meter of error.

[0055] The clock offset of GPS satellites is easily modeled by threeparameters. In the preferred embodiment, the measured clock offset ismodeled by the three parameters defined in ICD-GPS-200c. Theseparameters represent clock offset, drift, and drift rate. The parametersare adjusted in a similar way to the method 400 described above to givea model that best fits the measured data over the time interval.

[0056] Alternative embodiments may use longer fit intervals, such as 8,14, 26, 50, 74, 98, 122, or 146 hours for each ephemeris model. Thesefit intervals are envisaged in ICD-GPS-200c, but are seldom, if ever,available from the broadcast ephemeris. Under the current invention,models with these fit intervals may be generated even when the broadcastephemeris is limited to a 4-hour fit interval.

[0057] Alternative embodiments of the STD data may include observedsatellite velocity, acceleration, clock drift, or clock drift rate andthese terms may be used in the process of fitting a model in ways whichare well known in the art.

[0058] Another embodiment of an orbit model uses the spare data bits inthe current ephemeris format of a conventional GPS signal to provideadditional model parameters that would improve the data fit over longtime intervals. For example, subframe 1 has 87 spare bits that areavailable for additional parameters. This technique allows for moreparameters to describe the orbital motion of the satellites withoutcompromising the standard data format. This new ephemeris model is basedon the current ephemeris model with additional correction terms used toaugment the model to support the longer fit intervals with greateraccuracy.

[0059] Yet another embodiment of an orbit model is to develop a new setof orbital parameters that describe the satellite orbit which aredifferent, in part or in their entirety, from the GPS ephemeris modelparameters. With the goal of making the fit interval longer, differentparameters may provide a better description of the satellite orbit. Thisnew set of parameters could be defined such that they would fit into theexisting data structures, however, their implementation and algorithmsfor use would be different.

[0060] Still a further embodiment of an orbit model would be to developa new set of orbital parameters that would not fit into the existing GPSephemeris model format. This new set of parameters would be developed tobetter address the trade-off between the number of parameters required,the fit interval, and the orbit accuracy resulting from the model. Anexample of this type of ephemeris parameter set is Brouwer's theory thatcould be used as is or modified to account for GPS specific terms.Brouwer's theory as described in Brouwer, D. “Solution of the Problem ofArtificial Satellite Theory without Drag”, Astron J. 64: 378-397,November 1959 is limited to satellites in nearly circular orbits such asGPS satellites.

[0061] Another embodiment is to use a subset of the standard ephemerisparameters defined in ICD-GPS-200c. This approach is particularly usefulwhen bandwidth and/or packet size is limited in the communication linkthat will be used to convey the orbit model to the Remote GPS Receiver.In one such embodiment, the fifteen orbit parameters described above,and in ICD-GPS-200c, may be reduced to a subset of 9 parameters, bysetting all harmonic terms in the model to zero:

[0062] Square root of semi-major axis (meters^ ½)

[0063] Eccentricity (dimensionless)

[0064] Mean motion difference from computed value (radians/sec)

[0065] Mean anomaly at reference time (radians)

[0066] Longitude of ascending node of orbit plane at weekly epoch(radians)

[0067] Inclination angle at reference time (radians)

[0068] Rate of inclination angle (radians/sec)

[0069] Argument of perigee (radians)

[0070] Rate of right ascension (radians/sec)

[0071] Process 400 is then executed using this subset of parameters.This reduces the amount of data that must be sent to the Remote GPSReceiver. The receiver can then reconstruct a standard ephemeris modelby setting the “missing” harmonic terms to zero. There are a largenumber of alternative embodiments to reduce the size of the data, whilestill providing a model that fits the STD, including:

[0072] Removing parameters from the model, and replacing them with aconstant, such as zero—as done above—or some other predetermined value,which is either stored in the Remote GPS Receiver, or occasionally sentto the receiver.

[0073] The resolution of the parameters may be restricted in the process400, this too reduces the amount of data that must be sent to the mobileGPS receiver.

[0074] Parameters, which are similar among two or more satellites, maybe represented as a master value plus a delta, where the delta requiresfewer bits to encode; an example of this is the parameter Eccentricity,which changes very little among different GPS satellites.

[0075] Some of these approaches reduce the ability of the model to fitthe data over a period of time (e.g., six hours). In this case, the fitinterval may be reduced (e.g. to four hours) to compensate.

[0076] While the foregoing is directed to the preferred embodiment ofthe present invention, other and further embodiments of the inventionmay be devised without departing from the basic scope thereof, and thescope thereof is determined by the claims that follow.

1. A method for distributing satellite tracking data to a remotereceiver comprising: extracting at least a portion of satellite trackingdata from a memory; representing the at least a portion of satellitetracking data in a format supported by the remote receiver; andtransmitting the formatted data to the remote receiver.
 2. The method ofclaim 1 wherein said satellite tracking data comprises at least one of:a plurality of satellite positions with respect to time for a period oftime into the future, a plurality of satellite clock offsets withrespect to time for a period of time into the future.
 3. The method ofclaim 1 wherein said satellite tracking data comprises at least one of:data representative of satellite positions, velocities or acceleration;data representative of satellite clock offsets, drift, or drift rates.4. The method of claim 1 wherein said format comprises a format that isprescribed by said remote receiver.
 5. The method of claim 1 whereinsaid format is a model containing at least one of: orbital parametersand clock parameters.
 6. The method of claim 5 wherein said orbitalparameters and clock parameters are defined by the global positioningsystem standard.
 7. The method of claim 5 wherein said model comprisesmore than one sequential model, each sequential model being valid for aperiod of time.
 8. The method of claim 5 wherein said model is valid fora period of four hours.
 9. The method of claim 5 wherein said model isvalid for a period of more than four hours.
 10. The method of claim 1wherein said remote receiver is a GPS receiver.
 11. The method of claim1 wherein said remote receiver is a satellite positioning systemreceiver.
 12. The method of claim 1 wherein said format is a standardformat for transmitting satellite models to a global positioning systemreceiver.
 13. The method of claim 1 wherein the satellite tracking datais valid for a first period of time and the at least a portion of saidsatellite tracking data is valid for a second period of time, where saidfirst period is longer than said second period.
 14. The method of claim1 wherein said transmitting step further comprises: transmitting using awireless communications link.
 15. The method of claim 14 wherein saidtransmitting step further comprises: broadcasting the formatted data toa remote receiver.
 16. The method of claim 1 wherein said transmittingstep comprises: transmitting using a computer network.
 17. The method ofclaim 16 wherein said transmitting step further comprises: broadcastingthe formatted data to a remote receiver.
 18. The method of claim 1wherein said transmitting step comprises: transmitting using theInternet.
 19. The method of claim 18 wherein said transmitting stepfurther comprises: broadcasting the formatted data to a remote receiver.20. The method of claim 18 wherein said transmitting step couples theformatted data to the remote receiver when said remote receiver connectsto the internet.
 21. The method of claim 1, wherein said transmittingstep further comprises: determining a time when the cost of transmittingthe formatted data is relatively low; and transmitting the formatteddata at said time.
 22. The method of claim 1, wherein said transmittingstep further comprises: determining a time when the congestion of atransmission network is relatively low; transmitting the formatted dataat said time.
 23. A method of distributing satellite tracking data to aremote receiver, using the Internet, comprising: collecting saidsatellite tracking information at a tracking station; processing thesatellite tracking information to form satellite tracking data;formatting at least a portion of the satellite tracking data intoformatted data that is prescribed by a requirement of the remotereceiver; and sending said formatted data to the remote receiver. 24.The method of claim 23 where said satellite tracking information data isfrom at least one GPS satellite.
 25. The method of claim 24 where saidsatellite tracking information is at least a portion of the broadcastephemeris data from the at least one GPS satellite.
 26. The method ofclaim 23 wherein said sending step further comprises: broadcasting theformatted data to a remote receiver.
 27. Apparatus for distributingsatellite tracking data to a remote receiver comprising: a computer foraccessing at least a portion of satellite tracking data from a memoryand formatting the at least a portion of satellite tracking data in aformat supported by the remote receiver; and means for transmitting theformatted data to the remote receiver.
 28. The apparatus of claim 27wherein said satellite tracking data comprises at least one of: aplurality of satellite positions with respect to time for a period oftime into the future, a plurality of satellite clock offsets withrespect to time for a period of time into the future.
 29. The apparatusof claim 27 wherein said satellite tracking data comprises at least oneof: data representative of satellite positions, velocities oracceleration; data representative of satellite clock offsets, drift, ordrift rates.
 30. The apparatus of claim 27 wherein said format comprisesa format that is prescribed by said remote receiver.
 31. The apparatusof claim 27 wherein said format is a model containing at least one of:orbital parameters and clock parameters.
 32. The apparatus of claim 31wherein said orbital parameters and clock parameters are defined by theglobal positioning system standard.
 33. The apparatus of claim 31wherein said model comprises more than one sequential model, eachsequential model being valid for a period of time.
 34. The apparatus ofclaim 31 wherein said model is valid for a period of more than fourhours.
 35. The apparatus of claim 27 wherein said remote receiver is aGPS receiver.
 36. The apparatus of claim 27 wherein said remote receiveris a satellite positioning system receiver.
 37. The apparatus of claim27 wherein said format is a standard format for transmitting satellitemodels to a global positioning system receiver.
 38. The apparatus ofclaim 27 wherein the satellite tracking data is valid for a first periodof time and the at least a portion of said satellite tracking data isvalid for a second period of time, where said first period is longerthan said second period.
 39. The apparatus of claim 27 wherein saidtransmitting means comprises: a wireless communications link.
 40. Theapparatus of claim 27 wherein said transmitting means comprises: acomputer network.
 41. The apparatus of claim 27 wherein saidtransmitting means comprises: the Internet.